Disable DNS recursion and AXFR requests in BIND on Debian Linux and FreeBSD / How to test a nameserver if AXFR requests are allowed with dig command

Monday, 15th March 2010

I am playing with bind on a newly configured server and therefore doing my best to configure the nameserver in a good manner. In that manner of thoughts I remembered about the good old “recursion” which could pose a security hole in your DNS systems. I won’t buffle on how bad it is for a BIND domain resolver to have Domain recursion switched on, there is plenty of information you can read further online. Anyways here is a brief overview on recursion:
Recursive DNS is essentially the opposite of Custom DNS. Custom DNS is an authoritative DNS service that allows others to find your domain, and Recursive DNS allows you to resolve other people’s domains.

So considering the above definition if you decide to leave the default behaviour of the Bind nameserver (which by the way is also default behaviour of many other DNS servers including Microsoft DNS), this would mean that your DNS will be left open for the whole world to be able to serve resolve requests for any domain name requested by end users. In other words somebody out there might decide to use your nameserver to resolve all internet domains, like: google.com, yahoo.co.uk etc.

It is wise to enable recursion only for localhost on your bind name server, So to achieve that on Debian:
Open /etc/bind/named.conf.options and insert into it
Right before the options {

acl recurseallow {;; };

Also in the options {} include the following lines:

allow-recursion { recurseallow; };recursion yes;

On FreeBSD you need to include the same in /var/named/etc/namedb/named.conf by default or any other location if you have some specific named.conf file location.

Another truly Vital things to include in /etc/bind/named.conf.options on Debian Lenny among options {} is:

auth-nxdomain no;

Including this in the options {} configuration block would completely disable AXFR transfer requests on your nameserver on FreeBSD the procedure is absolutely analogous, just open /var/named/etc/namedb/named.conf and include the auth-nxdomain no; in the options configuration block.

To stress out the importance of disable AXFR it’s important to know that if you don’t disable the AXFR which is enabled by default in many nameservers out there you’re risking that a malicious person could list the whole zone files for each and every of the configured domains in the DNS server and consequently the attacker can learn a lot about the DNS topology of your network etc.
So to complete the article I’m gonna give an example on how the dig command can be used in order to check a certain DNS server if it has enabled the AXFR requests (e.g. if it’s vulnerable to this type of DNS information leak).

dig @somenameserver.net somedomainname.net axfr

In the above example somenameserver.net = is a random name server hosting a specific DNS domain
somedomainname.net = is the DNS domain name / (a.k.a. zone file) hosted on somenameserver.net

If everything is configured properly in your the namesever you’re running the axfr test against you should see something like:

; <<>> DiG 9.6.1-P1 <<>> @somenameserver.net somedomainname.net axfr
; (1 server found)
;; global options: +cmd
; Transfer failed.

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2 Responses to “Disable DNS recursion and AXFR requests in BIND on Debian Linux and FreeBSD / How to test a nameserver if AXFR requests are allowed with dig command”

  1. Panic says:
    Firefox 3.5.3 Firefox 3.5.3 Windows XP Windows XP
    Mozilla/5.0 (Windows; U; Windows NT 5.1; pl; rv: Gecko/20090824 Firefox/3.5.3

    Hi there may I use some of the information here in this post if I provide a link back to your site?

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    • admin says:
      Epiphany 2.29.92 Epiphany 2.29.92 Debian GNU/Linux Debian GNU/Linux
      Mozilla/5.0 (X11; U; Linux i686; en-us) AppleWebKit/531.2+ (KHTML, like Gecko) Safari/531.2+ Debian/squeeze/sid () Epiphany/2.29.92

      Of course go on, it’s my pleasure 🙂 I’m glad you found something useful here !

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